Macromolecules 2008, 41, 3049-3052
3049
Biocatalytically Synthesized Poly(3,4-ethylenedioxythiophene) Subhalakshmi Nagarajan,† Jayant Kumar,‡ Ferdinando F. Bruno,| Lynne A. Samuelson,| and Ramaswamy Nagarajan*,§ Departments of Chemistry, Physics, and Plastics Engineering, Center for AdVanced Materials, UniVersity of Massachusetts, Lowell, Lowell, Massachusetts 01854, and U.S Army Natick Soldier Research, DeVelopment & Engineering Center, Natick, Massachusetts 01760 ReceiVed August 7, 2007; ReVised Manuscript ReceiVed February 4, 2008
ABSTRACT: Over the past decade, poly(3,4-ethylenedioxythiophene) (PEDOT) has been one of the widely investigated conjugated polymers due to its excellent electro-optical properties. The conventional synthesis of PEDOT/PSS involves oxidation of EDOT using strong oxidants in aqueous polystyrenesulfonate (SPS) solution. The low pH conditions and strong oxidants render this synthetic protocol unsuitable for use of PEDOT in applications such as biosensing. For the purpose of expanding the utility of PEDOT in these applications, it is important to develop a route that can provide the possibility of synthesizing PEDOT in the presence of the appropriate biological entities. Here we report the use of terthiophene as a radical mediator to synthesize PEDOT/ PSS under milder pH conditions using soybean peroxidase (SBP). The oxidation potential of terthiophene is sufficiently low for initiation of the polymerization reaction catalyzed by SBP. The oxidized terthiophene helps the subsequent oxidation of EDOT, thus mediating the polymerization reaction. This novel approach involving the use of conjugated oligomers as redox mediators is generic and vastly expands the types of substrates (thiophenes, pyrroles) that can be polymerized using enzymatic methods and benign conditions.
Introduction The synthesis of solution-processable inherently conducting polymers continues to be of interest due to their ease of processing for electronic and photonic applications. Poly(3,4ethylenedioxythiophene) (PEDOT) is one of the successful and well-known π-conjugated polymers with good conducting and electro-optical properties.1 Apart from being synthesized from a monomer 3,4-ethylenedioxothiophene (EDOT) of relatively low oxidation potential (as compared to other thiophene monomers), PEDOT possesses a unique combination of properties and reasonably good stability in the oxidized state.2 PEDOT/ PSS has good film-forming properties, exhibits conductivities1 ranging between 10-3 and 10 S/cm, and is transparent in the visible wavelength region. PEDOT/PSS also exhibits electrochromic properties which have been demonstrated in devices.3 The possibility of using this electrically conducting polymer for biosensing4 (DNA sensors5) and drug delivery applications6 has already been explored. PEDOT has also been found to be superior to poly(pyrrole) for sensing applications.7 However, to date, most synthetic protocols used for the synthesis of PEDOT involves very low pH conditions and oxidants that are not compatible with biological systems. For the purpose of creating biosensors based on PEDOT, it is important to develop a synthetic route that can provide the possibility of synthesizing PEDOT in the presence of an appropriate biological entity. A biocatalytic approach could potentially pave the way to polymerizing EDOT in relatively benign conditions, rendering it suitable for a variety of biological applications. During the initial stages of development, PEDOT that was synthesized using the conventional oxidative chemical/electrochemical method was insoluble in water. However, the solubility problem was overcome using a water-soluble polyelectrolyte, * Corresponding author: phone 978-934-3454; e-mail
[email protected]. † Department of Chemistry, University of Massachusetts, Lowell. ‡ Department of Physics, University of Massachusetts, Lowell. § Department of Plastics Engineering, University of Massachusetts, Lowell. | U.S Army Natick Soldier Research, Development & Engineering Center, Natick, MA 01760.
poly(styrenesulfonic acid) (PSS). PSS was also found to help in keeping the PEDOT segments dispersed in water. PSS also acts as the charge-balancing dopant for PEDOT, thus providing the desired electrical properties.8 The most commonly used chemical method for the synthesis of PEDOT/PSS involves oxidation of EDOT by a strong oxidizing agent such as sodium persulfate in aqueous PSS solution.9 The resulting PEDOT/PSS is not truly water-soluble but forms a dispersion in water which is stable and processable. Experimental Section Materials. The thiophene monomers, EDOT, and terthiophene were obtained from Sigma-Aldrich Co. and were used without further purification. Peroxidase from Glycine max (soybean) (activity 108 purpurogallin units/mg), poly(sodium 4-styrenesulfonate) [PSSNa] (Mw ca. 70 000), PEDOT/PSS (1.3 wt % dispersion in water), and hydrogen peroxide (30% solution) were also obtained from Sigma-Aldrich Co. The hydrogen peroxide was diluted to 0.3% (in deionized water), and this solution was used for polymerization. Synthesis of PEDOT/PSSNa. The reactions were carried out in 80/20 (v/v) mixtures of citrate buffer (pH 3.5–5.5) and dimethyl sulfoxide (DMSO). The polymerization of EDOT in the presence of PSSNa and terthiophene was carried out using SBP and hydrogen peroxide under ambient conditions. 20.6 mg (10 mM) of PSSNa was dissolved in 7.8 mL of citrate buffer. A 1 mM stock solution of terthiophene was prepared by dissolving 2.48 mg in 10 mL of DMSO. A stock solution of SBP (12.5 mg/mL) was also prepared in deionized water. A typical reaction mixture contained 7.8 mL of citrate buffer containing PSSNa, 1 mL of DMSO containing 10.6 µL (10 mM) of EDOT monomer, 1 mL of the terthiophene stock solution, and 200 µL of SBP solution. The polymerization was initiated by the addition of 25 µL aliquots of 0.3% H2O2 solution. A total of 20 aliquots of the H2O2 solution were added at 3 min intervals to prevent inhibition of the enzyme by the H2O2 solution. The reaction mixture was stirred gently for several hours. The watersoluble complex was then transferred to individual regenerated natural cellulose membrane bags (molecular weight cutoff 1000 Da) and was dialyzed against 5000 mL of acidified deionized water maintained at pH 4.3. Dialysis was carried out for 72 h with fresh acidified deionized water being added every 6 h to expedite the removal of oligomers and unreacted monomer. The dry polymer
10.1021/ma0717845 CCC: $40.75 2008 American Chemical Society Published on Web 03/29/2008
3050 Nagarajan et al. was obtained by evaporation of solvent and drying at 40 °C under vacuum for 72 h. The gravimetric yield was around 78%. Characterization. Products were characterized using a PerkinElmer Lambda 9 UV–vis spectrometer. The FTIR measurements were carried out on films cast on a ZnSe disk by use of a Thermo Nicolet 370 FT-IR spectrometer. Thermogravimetric analysis (TGA) was performed using a TA Instruments Hi-Res 2950 thermogravimetric analyzer. The TGA of all samples were carried out in air. Conductivity measurements were carried out on PEDOT/PSS pressed pellet samples using a Cascade Microtech collinear fourpoint probe connected to a current source and electrometer. The conductivity values reported are the average of several readings at different regions and sides of the disk. Cyclic voltammetry (CV) experiments were performed under nitrogen at room temperature using lithium perchlorate as electrolyte in deionized water. Platinum wire was used as a counter electrode; Ag/AgCl was used as the reference electrode and glassy carbon as the working electrode. A scan rate of 100 mV/s was used and the potential of the working electrode was swept between -0.9 and 0.6 V vs Ag/AgCl. Under similar conditions, potassium ferrocyanide gave an oxidation peak at 0.384 V.
Macromolecules, Vol. 41, No. 9, 2008 Scheme 1. (a) Polymerization of EDOT Catalyzed by SBP Using Terthiophene As a Redox Mediator; (b) Proposed Mechanism for the Reaction
Results and Discussion Enzymatic Polymerization. In the recent years we have shown that the synthesis of conducting polymers such as polyaniline can be carried out enzymatically in the presence of biological entities like genomic DNA.10 Enzyme-catalyzed polymerizations are known to offer many advantages over traditional chemical approaches including environmentally friendly reaction conditions and ease of synthesis. Horseradish peroxidase (HRP) has been used for the oxidative polymerization of aniline in the presence of various charged macromolecular “templates” to yield water-soluble and conducting polyaniline complexes.11 However the catalytic use of HRP has been restricted to aniline-based monomers mainly due to the low redox potential [∼0.95 V with respect to standard hydrogen electrode (SHE) of the enzyme].12 In EDOT, the substituents in the 3- and 4-positions cause a decrease in the oxidation potential of the monomer13 to around 1.1 V, which is still significantly higher than the potential favorable for polymerization catalyzed by HRP. While there have been reports on the use of HRP for the polymerization of EDOT,14 these reactions proceed well only at low pH conditions and temperatures. Here we report the use of terthiophene as a radical mediator to synthesize PEDOT/PSSNa under milder pH and at ambient conditions using the enzyme soybean peroxidase (SBP) and poly(sodium p-styrenesulfonate) (PSSNa). SBP belongs to the family of plant peroxidases that can oxidize a wide variety of organic and inorganic substrates using hydrogen peroxide. SBP has been reported to have a higher redox potential as compared to HRP,15 but SBP cannot catalyze the polymerization of EDOT under standard conditions. However, we found that the polymerization of EDOT can be accomplished by introducing a small amount (
